1. Field of the Invention
This invention relates to a novel process for preparing 3,4,3',4'-benzophenonetetracarboxylic dianhydride (BTDA) which comprises condensing an impure ortho-xylene mixture with acetaldehyde in contact with an acid catalyst to obtain an impure 1,1-bis(3,4-dimethylphenyl)ethane (DXE) mixture, oxidizing the DXE mixture with nitric acid to obtain a mixture of carboxylic acids from which by crystallization and dehydration substantially pure BTDA is recovered.
2. Description of the Prior Art
ortho-Xylene, as well as other isomeric xylenes, are readily obtained from petroleum crudes through catalytic reforming or cracking as part of a C.sub.8 aromatic stream. A typical C.sub.8 aromatic distillate fraction from a refinery stream has the following composition:
TABLE I ______________________________________ Boiling Volume Per Cent Compound Point, .degree.C. Broad General ______________________________________ Components Boiling Below Ethylbenzene.sup.1 &lt;136 0 to 10 0 to 2 Ethylbenzene 136 12 to 30 15 to 25 para-Xylene 138 15 to 25 16 to 20 meta-Xylene 139 30 to 50 35 to 48 ortho-Xylene 144 12 to 30 12 to 28 Components Boiling Above ortho-Xylene.sup.2 &gt;144 0 to 10 0 to 5 ______________________________________ .sup.1 Mostly toluene .sup.2 Cumene, pseudocumene, mesitylene, etc.
By far the most important component commercially in the above distillate fraction is para-xylene, an important feedstock for the production of terephthalic acid. The second most important component, ortho-xylene, is a starting material for the production of phthalic anhydride via vapor phase oxidation.
Numerous literature references exist for the isolation of individual components present in the above mixture, for example, in U.S. Pat. Nos. 3,636,180 to D. B. Broughton, 3,653,184 to B. M. Drinkard et al, 3,707,550 to L. O. Stine, 3,715,409 to D. B. Broughton, 3,700,744 to V. Berger et al, etc.
In a typical procedure, for example a C.sub.8 aromatic distillate fraction, as defined in Table I, is separated into relatively pure components using a combination of molecular sieve adsorption, fractionation and isomerization steps. In the first, para-xylene and ethylbenzene are sorbed, leaving a raffinate of ortho- and meta-xylene. The sorbed components are desorbed with diethylbenzene, which is recovered by fractionation and ethylbenzene is separated from para-xylene in a second adsorption step. The sorbed para-xylene is desorbed with toluene, which is recovered by fractionation. The raffinate from each of the adsorption steps contain desorbent, which is recovered by fractionation. The ortho- and meta-xylene mixture is isomerized to obtain additional para-xylene and the isomerizate is fractionated to remove light ends and to recover a crude ortho-xylene fraction. A typical crude ortho-xylene fraction so obtained will have the following composition:
TABLE II ______________________________________ Volume Per Cent Compound Broad General ______________________________________ ortho-Xylene 88 to 98 94 to 96 meta-Xylene 0 to 3 0 to 2 para-Xylene 0 to 2 0 to 1 Ethylbenzene 0 to 1 0 to 1 Components Boiling Above ortho-Xylene 0 to 3 0 to 2 ______________________________________
The crude (or impure) ortho-xylene fraction so recovered requires no further purification for the synthesis of phthalic anhydride, since the other components present which are not converted to phthalic anhydride are degraded during the vapor phase oxidation to carbon dioxide and water. In the event an ortho-xylene fraction of higher purity is desired, for example, greater than 99 percent, typically 99.7 percent or better, the crude fraction is subjected to further distillation under more closely controlled and more efficient conditions involving additional expense. For over ten years, for reasons set forth below, ortho-xylene of such higher purity has been used commercially in operations leading to the production of 3,4,3',4'-benzophenonetetracarboxylic acid (BTA), an intermediate in the manufacture of BTDA.
It would appear to one skilled in the art that the most direct route for the preparation of BTDA would be through the oxidation of the corresponding bis(3,4-dimethylphenyl) methane (DXM I), since relatively inexpensive air or molecular oxygen could be used as the oxidant. See, for example, U.S. Pat. No. 3,652,598 to R. L. Broadhead. Furthermore, to convert the bridging methylene group (CH.sub.2) in DXM to a carbonyl bridge even if nitric acid were used as oxidant, would theoretically require only about 1.33 mols of nitric acid per mol of DXM as seen from the following equation: ##STR1## To convert each methyl substituent on the DXM molecule to a carboxyl group would theoretically require two mols of nitric acid as seen from the following equation: EQU --CH.sub.3 +2HNO.sub.3 .fwdarw.--COOH+2NO+2H.sub.2 O. (2)
Therefore to convert DXM to BTA using nitric acid as oxidant would theoretically require a total of 9.33 mols of nitric acid.
Unfortunately, in synthesizing DXM through the alkylation of ortho-xylene with formaldehyde only from about 78 to about 87 percent of DXM I, ##STR2## is obtained, with the remaining 13 to 27 percent being the isomers (3,4-dimethylphenyl-2,3-dimethylphenyl)methane (DXM II) ##STR3## and bis(2,3-dimethylphenyl)methane (DXM III) ##STR4## (See M. I. Farberov et al, Zhurnal Organicheskoi Khimii, Volume 4, No. 1, pages 163-178, 1968.)
The three isomers so obtained are next to impossible to separate from each other either at the hydrocarbon stage or after oxidation with nitric acid to acids. Therefore, after dehydration, an isomeric mixture of dianhydrides is obtained. We have found the dianhydride product so produced to be a low-melting solid, having a melting point around 150.degree. C., with the various dianhydride isomers exhibiting varying degrees of reactivity, and the properties of polymers produced therefrom, particularly polyimides, to be unacceptable. Thus, M. I. Farberov et al clearly state that ". . . for preparation of polymers isomeric anhydrides must not be present in these as impurities . . . " Additional problems that can be present during dehydration of the above oxidized mixture is the formation of lactones between a bridging carbonyl group and the carboxyl group located in the ortho position with respect to this carbonyl function, a significant problem in U.S. Pat. No. 3,652,598 to R. L. Broadhead.
The commercial production of BTDA has involved the nitric acid oxidation of DXE I. This is because it has been found that in condensing ortho-xylene with acetaldehyde to produce DXE I, the protonated acetaldehyde, because of its larger size compared to the corresponding protonated formaldehyde, reacts more slowly, and therefore more selectively, to give from about 98 to 99 percent of the desired DXE I isomer, ##STR5## and only from about one to two percent of 1,1-(3,4-dimethylphenyl-2,3-dimethylphenyl)ethane (DXE II), ##STR6## and 1,1-bis(2,3-dimethylphenyl)ethane (DXE III), ##STR7## (See M. I. Farberov et al referred to above).
Commercial oxidation of the above DXE mixture with nitric acid, requiring theoretically 3.33 mols of nitric acid per ethylidene group oxidation, or a total ##STR8## of 11.33 mols of nitric acid per mol of DXE, followed by dehydration of the resulting oxidation product, has led to substantially pure BTDA having a high melting point (226.degree. C.), and uniform reactivity. This material has been used in the preparation of polymers, for example, polyamide, having excellent properties.